We will develop a high-sensitivity photon-counting direct-conversion x-ray detector for digital mammography. By combining a high-sensitivity detector material, mercuric iodide, with newly available CMOS technology photon-counting readouts, we will achieve the ultimate in x-ray detector performance. The ability to count only those individual x-rays which fall into a prescribed energy window gives substantial performance advantages over old technology x-ray detectors where the total charge generated by all of the x-rays hitting a pixel is the only signal available. With the new technology we improve the signal to noise ratio by removing the dark current and readout circuitry noise. We can reduce the dose delivered and improve the system throughput without compromising image quality. This is important in digital mammography screening due to the large number of healthy patients who are examined. This advance in imaging performance is made possible by utilizing a new direct x-ray converter material, polycrystalline mercuric iodide, which is a high-Z large bandgap compound semiconductor. Because it has outstanding charge collection properties, the material can be used for photon counting. We grow the x-ray converter directly onto specially designed photon-counting readout devices. We take advantage of newly-developed CMOS readout technology, allowing us to place a charge-sensitive preamplifier, shaping amplifier, dual discriminators, and digital event counter for each pixel in the array, all within each pixel's geometric area. We expect to widely market the device as an OEM component to mammography system manufacturers. In Phase I of the project we will devise film deposition techniques that produce spectroscopic Hgl2, test the film performance on glass substrates, and then deposit the film on the CMOS readout devices and test for photon counting and imaging performance. In Phase II we will refine the design of the CMOS readout chip to allow a panel detector to be built up from detector modules, modify the Hgl2 deposition as required, and complete a mammography system panel detector prototype. Phase II testing of panel prototypes will be performed at both Photon Imaging and the UC Davis Medical Center.